Light-emitting diodes (LEDs) are attractive replacement candidates for conventional light sources based on incandescent bulbs and fluorescent light tubes. LEDs have higher energy conversion efficiency than incandescent lights and substantially longer lifetimes than both incandescent and fluorescent light fixtures. In addition, LED-based light fixtures do not require the high voltages associated with fluorescent lights.
In addition to providing a more cost-effective solution to the lighting problem, LED-based lighting systems have the potential of providing additional functionality that further increases the attractiveness of LEDs as replacements for conventional light systems. The light from an LED can be modulated at very high frequencies, since an LED can be turned on and off in a time of the order of a nano-second or less. Hence, LEDs can be used to send data by pulsing the LED to create a series of light pulses that represent the binary bits of the data that is being sent. Since the human eye cannot resolve these short light pulses, an observer will see only the average light intensity from the LED. Accordingly, a light source that is being utilized to light a room can, in principle, be used to transmit data to devices within the room as well without interfering with the lighting function in a manner that can be detected by a human observer.
Many LED light sources also include photodetectors and controllers that regulate the current through the LEDs to maintain the output at a specific color point and intensity. For example, systems in which the light source includes LEDs that generate different colors of light that are mixed to provide an output color that is maintained at a specified color point and intensity are known to the art. These light sources measure the output of the LEDs in a number of spectral bands and then adjust the intensity of light generated by the various LEDs to maintain the color point and intensity by controlling the average current through each LED. Since these devices already include both photodetectors and a controller, systems that utilize these components to further implement a data network have been suggested.
In such a system, the lighting unit also acts as a transponder for sending and receiving data. The data is sent by pulsing one or more of the LEDs. The LED that is pulsed can be one of the LEDs that is used to provide the lighting function or a separate LED that is reserved for the data transmission function. A photodetector in the light source acts as the receiver for the transponder. The photodiode can be a separate photodiode or one of the photodiodes used to monitor the output spectrum from the LEDs. In the latter case, the photodiode must also be able to view light that originates from sources outside the light source.
The data that is to be sent and the data that is to be received can be communicated over the power lines that are used to connect the light source to a power grid in a building. Systems that utilize the power grid as one component of a local area network in a building are known to the art. For example, these systems are used to provide wireless access points for the network. Analogous systems in which the lighting source provides the functionality of the wireless access point have been suggested. In such a system, a computer or other device would communicate on the network through an optical transponder that sends and receives data to the transponder in the lighting units within a room.
Such optically based local area network segments have the advantage of limiting the reception area to a specific room or area within that room. One problem with radio frequency network segments is the leakage of the signals into areas surrounding the area of interest within a building. The portion of the signal that escapes the building represents a security problem, as an eavesdropper can listen to the communications on the network or utilize the network assets without the permission of the network owner. To overcome the security problems, the signals must be encoded which increases the cost of the systems and computing overhead needed to communicate on the network.
In addition, the leaked signals make it difficult to implement another radio frequency based network in an adjacent area. The encrypted signal from one area appears to be noise in the adjacent area. In a commercial building in which a number of independent businesses are located in close proximity to one another, the effective range of a WiFi network can be reduced by the overlapping signals to the point that a single WiFi network cannot be implemented in one or more of the businesses. An optically based system, on the other hand, does not suffer from either the security problems or the noise problems.
One problem that is encountered in implementing such an optically based communication system is the large variation in signal strength that is encountered by the transponders. Consider a laptop computer operating in a room via a lighting system based optical network link. The laptop must include a light emitter and a light receiver that communicates with the transponder in the lighting system. The amount of light that is received by the light receiver in the laptop depends on the orientation of the receiver relative to the LED that is transmitting the signal in the overhead lighting system. In addition, the received light also depends on the orientation of the laptop receiver. Hence, the signal can vary over several orders of magnitude in intensity. This large range in intensities will interfere with the decoding of the signals unless some form of gain control is utilized to control the output of the receiver. Since cost is of prime importance in many of the devices that must communicate over the optical network link, any such gain control circuit is preferably implemented in conventional CMOS circuitry that can be incorporated in the controller used in the lighting system or in the controller used in the receiver in the consumer device.